ARTICLE pubs.acs.org/EF
Applicability of Cloud Point Depression to “Cold Flow” Mark R. Jemmett,* Milind Deo, Jessica Earl, and Patrick Mogenhan Department of Chemical Engineering, University of Utah, 50 South Central Campus Drive, Room 3290, Salt Lake City, Utah 84112, United States ABSTRACT: For “cold flow” and slurry (wax crystals present) flow conditions, temperature management of the oil and cooling apparatuses is tantamount to obtaining good, reliable data (Bidmus, H. O.; Mehrotra, A. K. Energy Fuels 2009, 23, 31843194). In the case of “cold flow”, isothermal conditions between the slurry oil and a cold section of pipe are needed (Merino-Garcia, D.; Correra, S. Pet. Sci. Technol. 2008, 26, 446); however, several studies have found that isothermal conditions are in fact not needed, although the reason for the multiple-degree discrepancy was not documented to be fully understood. This study seeks to explain, at least in part, a potential reason for this discrepancy in terms of how the oil itself is cooled and conditioned. A clear model oil containing 5% normal paraffin wax and 1.5% light vacuum gas oil (LVGO) was conditioned in a well-mixed, insulated reservoir and cooled using an internal cooling coil. In practice, the fluid in the coil must be colder than the oil target temperature to reach the said target in a timely and cost-effective manner. The theory is that warm oil in the reservoir, when subjected to the subcooled surface of the coil, experiences precipitation of waxes beyond that which is expected. Because of chemistry, surface area, and energy of crystallization, a hysteresis in precipitation and dissolving temperatures exists, thus preventing these waxes from readily going back into solution. In this study, sampling is direct from the reservoir at various conditions and wax crystals are removed using paper filtration under vacuum-pressure (10 psig) conditions to prevent inadvertent solubility changes within the solidliquid slurry mixture. The filtrate is then tested for initial cloud point using Fourier transform infrared (FTIR) spectroscopy and compared to the target oil temperature. The observed effect is a reduced filtrate cloud point between the target bulk temperature and coil temperature. This qualitative study suggests that, because of this reduction in the filtrate cloud point temperature, the temperature at which new precipitation and deposition will occur in a flowing system is reduced below what is expected. In fine, the simple act of cooling is distorting “cold flow” and other slurry flow results by reducing the component cloud point temperature of the oil.
’ INTRODUCTION Background. Paraffin waxes, large carbon chains, and structures present within petroleum crudes are a substantial source of lost revenue for the petroleum industry because these have the tendency to build up as deposits on pipeline walls. These deposits reduce the flow cross-sectional area, thereby reducing throughput and increasing the required pump duty necessary to maintain flow. Furthermore, in the case of pipeline shutdown for maintenance, these waxes crystallize under cooling conditions (such as hot oil subjected to cold subsea conditions) and can form cross-linked gels. These gels can stretch dozens of miles and require significant pumping pressure to restart in many cases; if the gel cannot be broken because of pipeline pressure limitations, the line must be abandoned or replaced at the cost of many millions of dollars. Many methods of paraffin wax deposition prevention and remediation have been explored over the years,38 ranging from pigging, chemical additives, heat tracing, coatings, biological measures, magnetics, and acoustics. While only pigging, chemical additives, and line heating have shown any real promise in the field and are unlikely to be completely replaced, these methods can be cumbersome and expensive, particularly in wells that are in difficult, extreme locations. In addition, the general attitude worldwide is that petroleum reserves in the future are only going to become more and more difficult to produce with regard to location and composition,9,10 whether it is asphaltenes, waxes, hydrate potential, acidity, or all issues combined. In the instance of paraffin waxes, a relatively new concept called “cold flow” has emerged as a means of mimimizing or r 2011 American Chemical Society
preventing wax deposition by eliminating the heat flux in an uninsulated pipeline1,2 or, in other words, eliminating a cold wall relative to the oil temperature. “Cold flow” is thus accomplished by reducing the temperature of the oil to that of the surroundings. For a crude oil with an initial wax appearance temperature (WAT) above ambient conditions, this would require that the state of the flowing oil be solidliquid slurry, with the solids being precipitated wax crystals. Multiple studies1,2 have shown that, while increasing pump duty because of raised viscosity, this “cold flow” method indefinitely prevents and eliminates deposition by pulling paraffin waxes out of solution, thereby eliminating its potential for nucleation and deposition on the walls, as well as eliminating the heat flux through the walls that introduces the deposition potential initially.1118 Furthermore, evidence in rheometric systems has shown that gels formed from slurry initial states are significantly weaker than gels formed from “clean”, pure-liquid oil states,19,20 giving “cold flow” one more benefit with future potential industrial application. With the most recent studies, a curious artifact was discovered with regard to the temperatures at which “cold flow” successfully functions. Carefully controlled laboratory flow systems have shown that, while the state of the oil is that of a slurry, ambient Special Issue: 12th International Conference on Petroleum Phase Behavior and Fouling Received: September 14, 2011 Revised: November 8, 2011 Published: November 11, 2011 2641
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Figure 1. Deposition experiments showing the pressure drop across a controlled test section. The inset shows extended results of longterm tests.
temperatures can actually be reduced a few degrees below that of the oil,1 effectively introducing a small but definitely existent heat flux across the wall. Theory and commercially available models dictate that if a flux is present and if wax is available in solution to deposit, waxes will eventually deposit in the absence of excessive shearing forces at the walls.8,1114,2128 However, in all cases and tests with this ambient temperature reduction, there was no such deposition. In the most recent study by the authors, using a clear heat exchanger, wherein deposition could be visually seen and measured, this result was confirmed: no deposition out of a slurry, with ambient temperatures being as much as 2 °C below the bulk oil temperature. A previous study1 came to a conclusion that part of the deposition elimination was due to the inner wall temperature being elevated above that of the coolant fluid, but as explained by the authors of that study, this is insufficient to explain the total loss of deposition and apparent reduction in actual WAT (also known as cloud point) or the temperature at which deposition can occur. The hypothesis is that some aspect of cooling the oil may be causing more wax than is expected for a particular bulk fluid temperature to come out of solution. As such, particular attention to the cooling coil for each test and bulk fluid temperatures is taken with regard to the samples and their subsequent WATs. It is important to note here that this resultant lack of deposition under a thermal gradient does not challenge or disprove in any way the prevailing and well-established theory of radial temperature gradient-driven deposition. Rather, the results of this study and the previous study1 appear to suggest that some kinetic, transient barrier is interfering with the thermodynamic driving force. Figure 1 presents several tests conducted by the authors, showing experimentally the effect just described. In the figure, an increase of the pressure drop across a controlled test section (Figure 3) indicates a restriction of flow because of deposition along the inside wall. The first test called “10 A, 17 O, 0.7 GPM”, which refers to an ambient temperature of 10 °C, an oil temperature of 17 °C, and a volumetric flow rate of 0.7 GPM, clearly shows an increase in the pressure drop from the beginning of the test. Reynold’s numbers for these tests ranged from 100 to 1000 (lamina r regime), with shear stresses at the wall comparable to deep-sea pipelines (920 Pa). The other two tests have a much smaller differential between ambient and oil temperatures; there is still a temperature gradient, and according to models and theory, deposition should occur. No pressure evidence arises, nor was there any visual evidence.
Figure 2. Schematic representation of forced cloud point depression.
Theory. Waxy crude petroleum (and, in this case, model oils) contains a wide spectrum of paraffinic components.29 WAT or cloud point generally indicates the initial WAT or temperature that shows the first solid precipitation. However, this is merely one of countless WATs (one for each paraffinic component in the crude mixture), and at each decreasing temperature, more and more compounds precipitate. Concerning wax crystals themselves, similar to other crystallization reactions, wax crystals give off small amounts of heat, being exothermic in nature. Heat must be added to these crystals to dissolve them back into solution, leading to a hysteresis between the WAT and wax disappearance temperature (WDT); for crude oils, the WDT is always higher than the WAT.30 A further source of this hysteresis is the reduced effective surface area for the melting reaction; a rigid crystal has less available reactive surface area than small, free molecules. The hypothesis arose when observing results during “cold flow” testing; just as in the study by Mehrotra et al.,1 the authors noticed that deposition was completely eliminated in a temperature-controlled test section even when ambient temperatures were several degrees below the flowing oil temperature. The arising hypothesis of this study dictates that the potential source of this anomaly was due to the conditioning or cooling process of the oil. The oil is subjected to a cooling medium that is colder than the intended bulk fluid target temperature during the cooling process. Heat losses drive this division, and from this division of temperatures, a thermal gradient emerges between the surface of the cooling medium (in this case, a cooling coil) and the bulk fluid, assumed to be an infinite medium (see Figure 2). As oil and pre-existing wax particles pass into this thermal gradient, paraffin precipitation occurs with lower carbon numbers than would normally precipitate at the bulk temperature. Existing crystals and other particulates provide the most favorable nucleation sites, with these providing a backbone or support as the crystals re-enter the bulk fluid. Because of the hysteresis between the WAT and WDT for these new crystals, dissolution does not readily occur, thereby effectively removing these lower carbon number paraffins from the liquid phase of the oil and, more importantly, preventing them from depositing normally under thermal gradients. We call this hypothesis “forced cloud point depression”, and this study aims to validate this hypothesis. In this study, filtered 2642
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Figure 3. Schematic of the flow loop. CantyVision uses a slipstream from the main flow line.
samples from an oil at sub-WAT conditions were analyzed and compared to the cooling coil and bulk fluid temperatures. If the hypothesis is supportable, then the WAT of the filtered samples should be found between the coil and bulk fluid temperatures, indicating that the cooling system is indeed affecting the propensity of the cooled oil to undergo deposition, as shown by Mehrotra et al.1 If the WAT is the same as the bulk fluid temperature or shows no recognizable pattern or relationship with regard to the bulk and coil temperatures, then this hypothesis would likely be faulty.
’ EXPERIMENTAL SECTION Apparatus Overview. The experimental setup used is shown in Figure 3. The system used in this study comprised of a heat-controlled recycled flow loop, unpressurized, with a large reservoir and customdesigned, clear, acrylic pipe-in-pipe heat exchanger with a length of 4 ft, with an internal pipe diameter of 1/2 in., and an outside pipe diameter (for coolant) of 11/2 in. This exchanger was originally designed for deposition testing; in this study, the exchanger is instead used to extend the difference between the coil and bulk fluid temperatures. The pump for the system was a progressive cavity type from Moyno, and heat control in the reservoir was similar to that of previous studies: a heavy insulator jacket with a 1/4 in. copper cooling coil filled with a flowing ethylene glycol/water mixture winding through the inner volume of oil. Mixing was performed using an IKA mixer motor and 3 in. impeller. All flow lines (with the exception of the pump loading lines) were 1/2 in. outer diameter stainless steel fitted with Swagelok fittings. The reservoir was chosen to be large in relation to the volume of the flow lines to minimize the effects of wax sequestration because of acrylic section deposition for its original purpose, but for this study, it functions well to minimize the depletion effects of sampling; nevertheless, the oil was checked for initial WAT and gel point after each day of testing to ensure constant fluid properties. Two cooling units, one for the reservoir and one for the acrylic exchanger, were used, with these being a programmable Julabo FP40 and Brinkman-Lauda RM20, both with very high flow rates for maximum efficiency. Cooling medium used was a 50:50 mixture of ethylene glycol and water, plus a mild biocide. Lastly, a specialized microcamera from JM Canty, Inc. was used to measure wax particle sizes through a slipstream. Testing Methods. Two types of tests were conducted as part of this study: static and flow. All testing involved cooling the oil under mixing conditions to steady state, sub-WAT, and above gel point temperatures;
this was achieved by selecting a fixed cooling coil temperature and recording the attributed steady-state bulk fluid temperature. During cooling, the coils inside the reservoir were manually cleaned using a brush in 5 min intervals to prevent wax depletion and heat-transfer reduction because of deposits. Static tests involve the oil being mixed in the reservoir without any outflow into the flow loop system. The flow test involves having the flow system fully operational, with the outflowing oil subject to ambient (25 °C) conditions, which are controlled using a separate and independent cooling system, which feeds coolant through the annulus of the test section. This type of test gave a larger spread between coil and bulk fluid temperatures because of heat being added to the flowing fluid from the surroundings. The operational and sampling procedures between the two test types were identical, with all flow conditions, such that the flow was laminar (Reynold’s number between 100 and 1000 and wall shear stresses between 9 and 20 Pa). Feedstocks. The oil used in this study was a model oil made with three basic components: Chevron Superla-7 mineral oil (92.5% by weight), light vacuum gas oil (LVGO) wax (6.0% by weight), and normal paraffinic Chevron wax (1.5% by weight). The LVGO wax is a unique blend of lower carbon number isomeric and normal alkanes and paraffins (Figure 4), having a melting point near 30 °C. The resultant stable mixture gives very desirable solidliquid equilibrium properties: WAT of 19 °C and gel point of 7 °C. Viscometrically, the oil is more viscous than typical crudes, but functionally, it gives a broad range of subWAT testing temperatures, something quite difficult to achieve with simple model oils. WAT was determined using the Roehner method31 with Fourier transform infrared (FTIR) spectroscopy. Solubility was roughly estimated using this same method (Figure 5), as well as a more accurate differential scanning calorimetry (DSC)-based method (Figure 6). The gel point was found using a controlled stress rheometer manufactured by TA Instruments. The gel point procedure was a common and effective cone-and-plate oscillatory sweep of 0.3 Pa at 0.06 Hz frequency (low shear as to not disturb gel formation), and the gel point itself was determined to be the point at which the storage modulus (G0 ) exceeded the loss modulus (G00 ).32 In practicality, this crossing point indicates that a forming gel is resisting deformation rather than being affected by the gentle motion of the cone. The DSC-based method shown in Figure 6 predicts continual precipitation well beyond 0 °C, consistent with the lower carbon number paraffin distribution of the largely LVGO wax blend shown in Figure 4. This cumulative integral method32 estimates the extent of precipitation against a known precipitable composition (in this case, the Chevron and LVGO waxes by weight percent). As seen in the figure, the data do not 2643
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Figure 4. Paraffinnic Chevron and LVGO wax carbon number distributions with combined wax distribution used in this study. “Wax” is the wax mixture composition. Mineral oil composition is not shown. This figure was prepared by the Research Partnership for Securing Energy for America (RPSEA).
Figure 5. Estimated solubility of the model oil using the FTIR spectroscopy method by Roehner et al. The slope of the solubility curve is steep at 0 °C, leading the authors to pursue DSC for a more realistic curvature.
Figure 6. Estimated solubility of the model oil using the DSC-based cumulative integral method. The dashed line represents the expected curve from data points.
follow a standard curvature; this is most likely due to instrumentation. A dashed line presenting the expectation of solubility by the authors based on the data is thus shown. Sampling Procedure. Sampling was performed using a vacuum filtration method, which was essentially “cold filtration”, as seen in Figure 7. This “cold filtration” system comprised of a Welch DuoSeal vacuum pump connected via a hose to a liquid catcher, which, in turn, was also connected to a special plastic filtration cup fitted with two layers of Whatman grade 50 filter paper (2.3 μm pore size) for complete solids removal. The sealed cup was nearly fully immersed in the reservoir of Figure 3 while sampling. After each test, the entire line was cleansed with warmed acetone to eliminate any traces of oil to prevent corruption. To ensure quality samples, the filtration cup was left inside the reservoir for 1 h to equilibrate with the steady-state reservoir fluid temperature. Sampling involved creating a vacuum pressure of 10 psig on the filtrated side of the filtration cup, after which fluid would slowly percolated up the filtration path. Vacuum pressure is kept low in these
tests to minimize any pressure-driven solubility issues. The filtrate was then required to build up until inside the vacuum tube until reaching the liquid catch, where 5 mL of sample was collected. It should be noted that the sampling procedure was slow, hence the need for a submersed, equilibrated filter system. As will be shown in Figures 8 and 9, particle sizes were estimated from videos made using an in-flow Cantyvision cross-polarized camera purchased from JM Canty, Inc. The particle size distribution for the tests in question ranged from 10 μm at the smallest to over 100 μm; initial testing showed that, even with this particle size distribution, some crystals were making their way through a single filter paper (cloudiness was apparent in the fluid catch), indicating that either some smaller particles existed in the fluid or particles were being sheared at the filter. Whichever the case, the result was the use of two filter papers to increase tortuosity, and this gave a satisfactory result and no detectable solids content. The shape of the crystals seen in the flowing system confirms the results of Fogler’s groups,32,33 which found wax crystals of at least model 2644
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Figure 7. Schematic for the “cold filtration” system.
Figure 10. Example of using the FTIR spectroscopy WAT analysis by Roehner et al. This particular static test was run with a coil temperature of 12.0 °C and a steady-state bulk reservoir temperature of 12.7 °C. The intersection point of the two slopes is approximately 12.4 °C.
Figure 8. Captured image from live flow video. Cross-polarized light through the Cantyvision camera causes wax crystals to light up as white shapes in a dark field. The flow temperature was 17 °C or 2 °C below the initial WAT.
Figure 11. Temperature profile for FTIR spectroscopy testing. The sample is cooled at 1 °C/min until 10 °C above the WAT, after which it is cooled at 0.1 °C/min. After the WAT test is complete, the sample is slowly reheated at the same rate and the estimated WDT is found.
Figure 9. Condensed wax particle size distribution from the video source of Figure 8. Note the bimodal nature of the distribution. oils to be ellipsoidal planes in the absence of smaller, high carbon number crystals. Furthermore, the bimodal distribution seems to indicate that clumping or sticking of wax particles is occurring, but that is more a topic for future study and not necessarily applicable to this study. Particle sizes were determined using similar flow conditions to those conducted in the experiments (i.e., Reynold’s number between 100 and 1000, wall shear stresses between 9 and 20 Pa, and a cooling rate of 0.1 °C/min in a well-mixed environment). Sample Analysis. Liquid samples were pulled from the “cold filtration” process and analyzed using the FTIR spectroscopy technique at the University of Utah for WAT analysis. To determine WAT of an oil using FTIR spectroscopy, we followed the method by Roehner et al.31 As a clear fluid cools, its capacity absorbance of particular wavelengths
increases with increasing density, but this change is linear with respect to the temperature. When phase change occurs (in this case, precipitation of wax crystals), the absorbance capacity suddenly increases beyond the fluid density, giving a steeper change in absorbance with respect to the dropping temperature. By integrating the area between these wavelengths, plotting the points on a graph (area versus temperature), and extrapolating and intersecting the two slopes linearly, as shown in Figure 10, one can estimate WAT to within reasonable error. An average of multiple runs for each sample was plotted and compared to the coil and bulk fluid temperatures inside the reservoir for the two testing cases; each temperature yielded two samples to improve confidence. In the case of the filtrate samples, at cooler temperatures, one would expect the filtrate to have a lower actual WAT because more wax will have been precipitated in the bulk fluid. To prevent supersaturation issues (suppression of solid precipitates because of the lack of nucleation sites), the cooling rate from ∼10 °C above the WAT (30 °C) to the lower limit of 3 °C was set to be 0.1 °C/min; cooling from the conditioning temperature of 6030 °C was set to be much higher at 1 °C/min. The WDT is found in the same manner, only while heating the cooled sample. The heating/cooling procedure is shown graphically in Figure 11. The FTIR spectroscopy system used was a Perkin-Elmer Spectrum RXI, with a custom-built, temperature-controlled liquid sample cell. 2645
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Table 1. Summary of the Static and Flow Tests Using “Cold Filtration” test
condition
coil
bulk t
method
name
temperaturea
emperature
WATb
WDTb
F8 F9
10.0 10.0
10.8 10.8
10.2 10.2
12.4 13.6
F10
15.0
15.5
15.3
16.7
F11
12.0
12.7
12.4
13.9
F12
8.0
8.9
8.6
11.5
FL1
15.0
17.4
15.5
18.3
FL2
13.0
15.9
15.3
16.6
FL3
6.0
10.1
9.6
10.9
FL4 FL5c
10.0 9.6
13.5 13.5
12.2 12.0
13.7 13.6
static
flow
a
All temperatures were reported in degrees Celsius. b An accepted error of (0.5 °C was assumed for all data based on FTIR spectroscopy accuracy. c FL5 featured a test section jacket of 13.5 °C. All other flow tests were with a 25 °C jacket.
Figure 12. Static method test results. The WDT results are shown for comparison.
The temperature was maintained using an externally controlled Julabo chiller.
’ RESULTS AND DISCUSSION Summary of Experiments. Static and flow tests were conducted with the coil temperature ranging from 15 to 6 °C. Table 1 presents a summary of the test results. All flow tests were conducted using the same flow rate of 1.0 gpm (the resultant Reynold’s number was approximately 200), with a wall shear stress of approximately 17 Pa (comparable to deep-sea pipeline conditions). The next two sections will deal specifically with the static and flow results. It should be mentioned again that, while general solubility is largely independent of the cooling and heating rates (aside from supersaturation effects), the crystal size and rate of formation are heavily dependent. Furthermore, with very high heating rates (>1 °C/min), one can encounter heat-transfer limitation, which can skew the WAT and WDT results. As such, a low cooling rate of 0.1 °C/min (one similar to those found in the field) was chosen. The authors recognize that for a well-mixed and agitated system, supersaturation is practically non-existent; however, for consistency and to eliminate any unforeseen issues, this rate was chosen for both cooling in the flow loop and in FTIR spectroscopy. Static Testing Method. The results of the static method tests are presented in Figure 12. The temperature difference ΔT (bulk coil) was small, being on the same order as the error in the WAT measurement. This small difference is due to the excellent insulation of the reservoir that, in other types of experiments, is very beneficial in combating heat loss. Nevertheless, in all cases, the measured WAT values were found to be between the coil and bulk temperatures. Flow Testing Method. Operating the flow loop while cooling broadened ΔT for all tests, giving the resultant WAT values with their corresponding errors a much better resolution. These results are presented in Figure 13. All tests were conducted using a flow rate of 1 gpm through a very-low-shear progressive cavity pump, with test section conditions held at 25 °C (except for test FL5) to prevent in-line deposition. As a comparison, test condition FL5 generated samples while holding the test section
Figure 13. Flow method test results.
jacket at 13.5 °C to mimic a “cold flow” condition. The resulting bulk temperature, WAT, and WDT appear to be largely unaffected by the change in the jacket temperature. Curiously, the coil temperature required to maintain the 13.5 °C bulk fluid temperature was lower by 0.4 °C than the similar FL4 condition; it is suspected that the ambient temperatures of the summer test may have been slightly warmer than the FL4 condition. As a point of clarification, the flow tests (and all tests pertinent to this study) were not conducted to determine the test section deposition rates or presence. In fact, the test section was maintained with a coolant tempeature above the oil temperature, thus totally preventing any deposition from occurring. As mentioned earlier, the purpose of using the test section was for convenience; by doing so, the authors were able to extend the temperature difference between the cooling coils of the reservoir and the builk oil temperature, thereby giving improved resolution in the WAT results. Discussion. The static tests indicate that the WAT of the filtered samples lies between the coil and bulk temperatures. There is however a small difference in ΔT and relatively larger error. The flow tests, on the other hand, show very clearly, regardless of error, that the WAT of the filtered samples is 2646
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Energy & Fuels considerably below the bulk reservoir fluid temperatures in all cases. It may seem that there are multiple thermodynamic equilibria (distinctly different WAT and WDT values), but that is scientifically unfeasible. What the authors supect is that, rather than the difference being due to a thermodynamic limitation, the difference is likely a kinetic effect caused by the crystalline structure of wax limiting the phase transition. In other words, the WAT and WDT are not fixed values but rather the apparent steady states because of this kinetic limitation. If an infinite amount of time were given in the heating and cooling of the samples, these two values would be the same. However, industry does not operate on infinite time scales, and as such, the hysteresis is observed. This was an experimental study to evaluate a potential source of the “cold flow” anomaly; the evidence found through this study appears to confirm, at least in part, the forced cloud point depression hypothesis. While the mechanism of crystallization and the physical parameters and properties are as of yet undetermined, the results show a definite effect of the cooling system on the dissolved paraffin content of the sub-WAT oil. This effect could be caused by the transient, kinetic-based hysteresis of the solubility of the wax crystals; if true, once precipitated, it would necessarily take more energy and/or time than is available in the system at these testing regimes to melt them back into solution and thermal equilibrium.
’ CONCLUSION This study was an examination of the phenomenon of “cold flow” that allows the ambient conditions to be lower than fluid conditions in laboratory flow systems. This effect has been seen in multiple studies using varied and different geometries and cooling systems. To explore the reasoning behind the elimination of deposition in a pipeline with a finite heat flux through the walls, a series of tests of both a static and flowing system were conducted at various sub-WAT conditions for a designed model oil. With wax crystals carefully filtered out without heat addition, FTIR spectroscopy analyses on these samples confirmed that the actual WAT of the samples or the temperature at which deposition could occur was markedly reduced below the bulk oil temperature, most visibly in the flow method samples. All tests showed the same pattern: measured WAT values, within error, were found to be between the measured bulk fluid and cooling coil temperatures. This result seems to confirm the forced cloud point depression hypothesis, which states that the conditioning of an oil below the initial WAT in a real-world system can reduce the available fractions of paraffin beyond what the operator expects and that this occurrence is due to the hysteresis between the WAT and WDT. While geared primarily toward laboratory studies of crude oils in flow loops, there is a potential industrial application with pipelines. Simply pulling crystals out of solution by flash cooling or other similar method appears to be enough to temporarily block paraffin waxes from depositing on pipeline walls. In other words, it may be possible to implement the results of “cold flow” without actually having to fully reach isothermal conditions. It is recognized that significant modifications will be necessary for the commercial process. ’ AUTHOR INFORMATION Corresponding Author
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dx.doi.org/10.1021/ef2013908 |Energy Fuels 2012, 26, 2641–2647
